Aromatic amines are important intermediates which have to be available cheaply and in large amounts. Plants having very large capacities therefore have to be built for, for example, the hydrogenation of dinitrotoluene (hereinafter also referred to as DNT). The hydrogenation product toluenediamine (hereinafter also referred to as TDA) is an important intermediate in the preparation of tolylene diisocyanate which is of great importance in polyurethane chemistry. There are numerous publications relating to the preparation of toluenediamine. By far the largest part of the prior art is concerned with the hydrogenation of dinitrotoluene in the liquid phase. Known processes include a “single-phase” process, either without further solvent (see, for example, U.S. Pat. No. 3,093,685) or using solvents which dissolve both DNT and the TDA/water mixture formed, for example simple aliphatic alcohols (e.g. methanol). In addition, there are also “two-phase” processes in which solvents (e.g. hydrocarbons) which dissolve DNT but not the TDA/water mixture formed are used, so that phase separation occurs; see, for example, GB 1 490 313. The catalyst (for example Pd/C, Raney Ni, Ni/SiO2, etc.) is usually slurried in the liquid phase (therefore also referred to as “slurry-phase process”). Possible reactors are, for example, loop reactors or stirred vessels (see, for example, US 2011/295039 A1). All industrially relevant processes at present work in the liquid phase. Liquid-phase hydrogenation processes at elevated temperatures and the gas-phase hydrogenation of dinitrotoluene do not play any role industrially because of the potential hazards resulting from the thermal instability of, in particular, technical-grade dinitrotoluene. The gas-phase hydrogenation of nitroaromatics having little volatility and/or temperature sensitivity is considered to be critical in the literature (see, for example, Cartolano, A. R. and Vedage, G. A., 2004, Amines by Reduction, Kirk-Othmer, Encyclopedia of Chemical Technology, John Wiley & Sons Inc.; 5th edition (Jan. 31, 2004), Vol. 2, page 478 and page 484, Online-ISBN: 9780471238966).
GB 599,252 and U.S. Pat. No. 3,136,818 describe processes for preparing aromatic monoamines, in particular aniline, in the gas phase by hydrogenation in a fluidized-bed reactor. Since mononitroaromatics are substantially more stable than dinitroaromatics, uncontrolled thermal decomposition does not present a significant problem. GB 599,252 explicitly warns against the gas-phase hydrogenation of starting materials having relatively high proportions of dinitro compounds.
DE-B 1 809 711 is concerned with a process for the gas-phase hydrogenation of nitro compounds and in particular addresses the problem of uniform introduction of liquid nitro compounds into a hot gas stream by atomization, preferably at constricted places directly upstream of the reactor. The danger of possibly incomplete vaporization of the nitro compound is not mentioned. Although this document speaks in general terms of nitro compounds, it gives a specific example only for nitrobenzene. The process parameters mentioned in the document are optimized for nitrobenzene.
DE-A 3 636 984 describes a process for the coupled production of nitroaromatics and dinitroaromatics from the corresponding hydrocarbons by nitration and subsequent hydrogenation. The hydrogenation is carried out in the gas phase at temperatures of from 176 to 343.5° C. A description is given of an apparatus for the gas-phase hydrogenation which consists essentially of two reactors connected in series with intermediate cooling and intermediate introduction of starting material, but nothing is said about the size and structure of these. The problems of decomposition of dinitrotoluene is not addressed in the document.
The documents EP 0 696 573 A1, EP 0 696 574 A1, EP 0 748 789 A1, EP 0 748 790 A1 and DE 10 2006 035 203 A1 are concerned with a gas-phase process for the hydrogenation of aromatic nitro compounds which is carried out under purely adiabatic conditions. EP 0696574 A1 describes the process for preparing aromatic amines, in which a gas mixture consisting of nitroaromatics and hydrogen is passed over the catalyst under adiabatic conditions, in a quite general way. According to the other documents mentioned, particular advantages are in each case achieved in the processes by changing various parameters. The processes are applicable to nitroaromatics of the general formula
where R2 and R3 can be, inter alia, a methyl group. However, the focus of said documents is on aniline (the examples are concerned with the hydrogenation of nitrobenzene). The documents mentioned do not go into particular aspects of the hydrogenation of dinitrotoluene.
GB 832,939 is concerned specifically with the hydrogenation of dinitro compounds in the gas phase. This document discloses the use of nickel sulfide catalysts on an aluminum oxide support material. According to the document, the use of these makes an unexpectedly rapid reaction in excellent yields possible. The document does not go into process engineering details in respect of vaporization. The hydrogenation is carried out at ambient pressure and temperatures of about 220° C. (cf. examples), i.e. below the decomposition temperature of liquid pure dinitrotoluene.
DE 3734344 A1 describes the conversion of dinitrotoluene (DNT) in the gas phase into toluenediamine (TDA). DNT is vaporized in an inert, hot carrier gas within from 2 to 120 seconds to give a mixture which is composed of vaporized DNT and carrier gas and has a temperature of from 150 to 250° C. As suitable types of vaporizer, mention is made of thin film evaporators having smooth tubes, short path evaporators, falling film evaporators without circulation of liquid and single-coil helically coiled tube evaporators. The low volatility and ready decomposition of DNT and the explosion risk associated therewith are mentioned. Measures for avoiding decomposition or for avoiding accumulation of high-boiling, thermally sensitive impurities are not described since they presumably were not a problem because of the short time of the experiment of a few hours and possibly because of the purity of the starting materials used in the experiments carried out. The possible presence of nonvolatile components in the DNT is mentioned only insofar as the vaporization process can theoretically be used for separation into DNT and nonvolatile components. The hydrogenation is, according to this document, carried out in the temperature range from 200 to 450° C. and preferably at atmospheric pressure.
It is not possible to derive any technical concept for an economical reaction on a large scale from the literature sources which expressly refer to the possibility of hydrogenating DNT in the gas phase. The literature (in particular GB 832939 and DE 3734344 A1) does not address either the hurdles to be overcome in implementing an industrial DNT hydrogenation process or the utilization of the advantages potentially associated with such a process.
In respect of the industrial production of TDA by a gas-phase process, it has to be noted that DNT of technical-grade purity can have a higher proportion of relatively nonvolatile accompanying components than DNT which is used in small batches for laboratory experiments. For economic reasons, it is desirable to be able to use technical-grade DNT without complicated and expensive prepurification. This requires particular measures in the vaporization of DNT in industrial production plants.
None of the abovementioned documents gives any indication that systematic steps have been undertaken in order to allow safe vaporization and gas-phase hydrogenation of dinitrotoluene of technical-grade purity for long periods of operation and on an industrial scale, without DNT or its accompanying components decomposing in an uncontrolled manner. Little information is likewise given about                a) critical temperature limits for avoidance of thermal decomposition of DNT,        b) handling accompanying materials which vaporize with difficulty or not at all and have a hazard potential (e.g. picric acid, cresols, trinitrotoluene (TNT)),        c) avoidance of the accumulation of unvaporized materials which have a hazard potential in the unvaporized state (DNT, TNT, etc.) and        d) ensuring complete conversion of DNT before the product gas stream is cooled.        
The hydrogenation of DNT liberates large quantities of energy. In the conventional liquid-phase process, hydrogen is introduced at an absolute pressure of from about 20 to 100 bar and the reactors are operated at this pressure; see, for example, US 2008/0146847 A1 (100 bar pressure and a temperature of 150° C.). Owing to the low temperature level, utilization of the energy to be removed is possible and/or economical to only a very limited extent. If the hydrogenation reaction were to be able to be carried out at a higher temperature so that higher-pressure steam were to be able to be generated, this would have great economic value. This applies particularly to integrated systems comprising a plurality of production plants in which steam obtained in one process can be utilized in other processes (for example for heating the starting materials to the reaction temperature). Recently, carrying out the liquid-phase hydrogenation without solvent at a temperature which allows the production of steam at a pressure level of 4 bar has been reported (US 2011/0275858 A1). For this purpose, the hydrogenation was carried out at a temperature of 185° C., which was possible in a safe manner under the following conditions:                1. A reactor having an internal heat-exchange surface and an external circuit with removal of heat was used.        2. DNT was introduced into the catalyst suspension by means of a driving nozzle below the surface of the liquid.        3. The average DNT concentration in the reactor was limited to a value of less than 1000 ppm.        4. The hydrogen concentration including the hydrogen in the external circuit was set to a value of greater than 1% by volume, preferably greater than 3% by volume.        
Adherence to these conditions is essential to the process described, because, inter alia, nitro and nitroso compounds can decompose explosively in the presence of TDA at elevated temperatures (DE 10 2005 008 613 A1). However, the third and fourth conditions in particular can in the case of industrial production at a temperature of 185° C. or above lead to great practical problems in the design and operation of a liquid-phase process. Matching of the individual parameters to one another is difficult to realize. As will be explained in more detail below, the present invention makes it possible to maintain a temperature of 185° C. or above so that high-pressure steam can be obtained in a gas-phase process without comparable practical limitations.
In addition, gas-phase processes have a series of other advantages. Thus, separation of the catalyst from the product is easier since the product leaves the reactor in gaseous form while the catalyst remains in the reactor. Scale-up of the process is also simpler in the case of gas-phase reactors than in the case of the reactors customary in the slurry-phase process. Since mechanical stirring is not necessary in the gas-phase process, the risk of plant downtimes due to caking on the stirrer and also the energy consumption is lower. In addition, gas-phase reactors are simpler to clean than stirred vessels.
A further advantage of an industrial TDA synthesis in the gas phase arises from the possibility of subjecting the gaseous product to a prefractionation (including isomer separation) in a simple way by fractional condensation at different temperatures. Each of the condensate fractions obtained in this way can be fed in at a different place matched precisely to this fraction in the subsequent distillation sequence. This gives the possibility of considerably simplifying the distillation compared to the customary distillation of the product from a liquid-phase process (up to six columns, see, for example, U.S. Pat. No. 6,359,177 B1).
There was therefore a need to provide a process for carrying out the hydrogenation of DNT of technical-grade purity in the gas phase, which can be implemented on an industrial scale and thus make it possible to actually make use of the many advantages of a gas-phase hydrogenation, in particular the opportunity of obtaining high-pressure steam and the simplified work-up by fractional condensation, and thereby to significantly increase the energy efficiency of the process. The particular challenge was to avoid the risk of uncontrolled decomposition of DNT and its accompanying components.